Re: [RFC 12/20] iommu/iommufd: Add IOMMU_CHECK_EXTENSION

[Jason Gunthorpe]

On Sun, Sep 19, 2021 at 02:38:40PM +0800, Liu Yi L wrote:
> As aforementioned, userspace should check extension for what formats
> can be specified when allocating an IOASID. This patch adds such
> interface for userspace. In this RFC, iommufd reports EXT_MAP_TYPE1V2
> support and no no-snoop support yet.
> 
> Signed-off-by: Liu Yi L <yi.l.liu@xxxxxxxxx>
>  drivers/iommu/iommufd/iommufd.c |  7 +++++++
>  include/uapi/linux/iommu.h      | 27 +++++++++++++++++++++++++++
>  2 files changed, 34 insertions(+)
> 
> diff --git a/drivers/iommu/iommufd/iommufd.c b/drivers/iommu/iommufd/iommufd.c
> index 4839f128b24a..e45d76359e34 100644
> +++ b/drivers/iommu/iommufd/iommufd.c
> @@ -306,6 +306,13 @@ static long iommufd_fops_unl_ioctl(struct file *filep,
>  		return ret;
>  
>  	switch (cmd) {
> +	case IOMMU_CHECK_EXTENSION:
> +		switch (arg) {
> +		case EXT_MAP_TYPE1V2:
> +			return 1;
> +		default:
> +			return 0;
> +		}
>  	case IOMMU_DEVICE_GET_INFO:
>  		ret = iommufd_get_device_info(ictx, arg);
>  		break;
> diff --git a/include/uapi/linux/iommu.h b/include/uapi/linux/iommu.h
> index 5cbd300eb0ee..49731be71213 100644
> +++ b/include/uapi/linux/iommu.h
> @@ -14,6 +14,33 @@
>  #define IOMMU_TYPE	(';')
>  #define IOMMU_BASE	100
>  
> +/*
> + * IOMMU_CHECK_EXTENSION - _IO(IOMMU_TYPE, IOMMU_BASE + 0)
> + *
> + * Check whether an uAPI extension is supported.
> + *
> + * It's unlikely that all planned capabilities in IOMMU fd will be ready
> + * in one breath. User should check which uAPI extension is supported
> + * according to its intended usage.
> + *
> + * A rough list of possible extensions may include:
> + *
> + *	- EXT_MAP_TYPE1V2 for vfio type1v2 map semantics;
> + *	- EXT_DMA_NO_SNOOP for no-snoop DMA support;
> + *	- EXT_MAP_NEWTYPE for an enhanced map semantics;
> + *	- EXT_MULTIDEV_GROUP for 1:N iommu group;
> + *	- EXT_IOASID_NESTING for what the name stands;
> + *	- EXT_USER_PAGE_TABLE for user managed page table;
> + *	- EXT_USER_PASID_TABLE for user managed PASID table;
> + *	- EXT_DIRTY_TRACKING for tracking pages dirtied by DMA;
> + *	- ...
> + *
> + * Return: 0 if not supported, 1 if supported.
> + */
> +#define EXT_MAP_TYPE1V2		1
> +#define EXT_DMA_NO_SNOOP	2
> +#define IOMMU_CHECK_EXTENSION	_IO(IOMMU_TYPE, IOMMU_BASE + 0)

I generally advocate for a 'try and fail' approach to discovering
compatibility.

If that doesn't work for the userspace then a query to return a
generic capability flag is the next best idea. Each flag should
clearly define what 'try and fail' it is talking about

Eg dma_no_snoop is about creating an IOS with flag NO SNOOP set

TYPE1V2 seems like nonsense

Not sure about the others.

IOW, this should recast to a generic 'query capabilities' IOCTL

Jason